In the radio communication field, as a transmitter that accurately controls an emission direction of a radio wave without mechanically changing the direction and a receiver that accurately detects an arrival direction of a radio wave without mechanically changing the direction, an active phased array transmitter and an active phased array receiver are known. In addition, as a transceiver that has the transmission function and the reception function, an active phased array transceiver is known. First, the active phased array transmitter is described as an example.
The active phased array transmitter has an array antenna including a plurality of transmission antennas formed in an integrated manner in an array and spatially synthesizes radio waves emitted from the plurality of transmission antennas. The active phased array transmitter has a plurality of transmitting units connected to the plurality of transmission antennas, a transmission frequency signal source for generating a high-frequency transmission frequency signal, and a modulator that modulates the transmission frequency signal based on transmission data and generates the modulated transmission signal. The modulated transmission signal is supplied to the plurality of transmitting units. The transmitting units has variable gain amplifiers and phase shifters, respectively. The transmitting units independently change the amplitude and phase of the supplied modulated transmission signal. Signals paths formed by the transmitting units are referred to as channels.
The array antenna may control emission directions of radio waves to be output from the array antenna by controlling the signal of the channels so that the amplitudes and phases of the signal of the channels match in a certain direction. However, if the modulated transmission signal has a significantly high frequency or a millimeter wave is used, a variation in characteristics of circuits on which the transmitting units are mounted is large, the transmitting units are largely affected by a temperature fluctuation, the circuit characteristics vary, the amplitudes and phases of the output signal of the channels may not be controlled with high accuracy, and the actual directionality and the emission directions may not be desirable.
An active phased array transmitter that has a calibration function with a configuration for detecting the amplitude and phase of an output signal for each of channels has been proposed. The active phased array transmitter has a circuit for detecting the amplitude and phase of the output signal at positions close to transmission antennas for the channels. This circuit detects the amplitude and phase of the output signal of the transmission channels on a channel basis and corrects amplifiers and phase shifters for the transmission channels so that detected differences between the channels are reduced to zero. Specifically, a coupler is arranged close to the transmission antennas, the transmission signal is output from only a single channel, and the coupler generates a reception signal corresponding to the transmission signal of the single channel. A mixer mixes the reception signal with a local transmission frequency signal having a frequency slightly different from a transmission frequency signal so that the reception signal is converted to an intermediate frequency signal with a low frequency. The intermediate frequency signal is converted from analog to digital and subjected to a fast Fourier transformation (FFT) process by digital processing, and the amplitude and phase of the intermediate frequency signal are detected. By executing this process on all the channels, a variation in differences between the amplitudes of the signal of the channels and a variation in differences between the phases of the signal of the channels may be detected. Based on the results of the detection, the amplifiers and phase shifters of the transmitting units (channels) are controlled and the amplitude and phase of the signal are reset. Actually, this cycle is executed multiple times, and calibration is executed so that variations in differences between the amplitudes and phases of radio waves from the multiple transmission antennas are equal to or lower than defined values.
In the aforementioned technique, the transmission frequency signal and the local transmission frequency signal that are high frequency signals are generated by independent signal sources. Thus, uncorrelated jitter caused by phase noise is superimposed on the transmission frequency signal and the local transmission frequency signal. When the mixer generates the low-frequency intermediate frequency signal from the transmission frequency signal and the local transmission frequency signal, large jitter occurs to the intermediate frequency signal. Thus, there is a problem that the accuracy of the amplitude and phase of the detected intermediate frequency signal is low and that the accuracy of the calibration is low. Not only the active phased array transmitter but also the active phased array receiver and the active phased array transceiver have the same problem as described above.
The followings are reference documents.
[Document 1] Japanese National Publication of International Patent Application No. 2011-507000,
[Document 2] International Publication Pamphlet No. WO2004/109952, and
[Document 3] Japanese National Publication of International Patent Application No. 10-503892.